I have two tables with the same columns, and I need to copy one table's rows to the other table's rows to create one big table with all the values from both tables. Right now I am doing this query to return the same thing:
SELECT col1, col2, col3 from Table1
union
SELECT col1, col2, col3 from Table2
However, it seems horribly inefficient, and on my system is very slow (returns 1210189 records).
May it work to just do:
SELECT col1, col2, col3
INTO Table1
FROM Table2
Start with union all:
select col1, col2, col3 from Table1
union all
select col1, col2, col3 from Table2
Your query is trying to deduplicate things, which would slow it down considerably.
I think the best option is to create a view in sql server, this will optimize the performance of the query:
SELECT col1, col2, col3 from Table1
union all
SELECT col1, col2, col3 from Table2
(As other users said: "union" is used to select distinct values from two tables
where as "union all" is used to select all values including
duplicates from the tables.)
At the same time I would restrict the number of rows I get from the database if i am writing them for a web and if this is giving me problems, with the new functions of Sql Server 2005 row_number(), with this I would page results.
You could use this to fill the second table:
Insert into table2 select * from table1;
Or if you want to be more specific:
Insert into table2(col1, col2, col3) select col1, col2, col3 from table1;
(Note: some DBMSs might require putting parenthesis around the SELECT clause.)
select * into new table(your new table name)
from table1.col1,table1.col2,table2.col1;
here columns can be your required columns .
select * into newtable from table1
union all
select * from table2
Worked well. Guidelines, both tables have exact same column names :)
Related
I have two tables A & B. They share some columns. I want to select the common part of these two tables.
I know the following code is ok:
select col1, col2, ..., colN from A
union all
select col1, col2, ..., colN from B
However, I think the code is not good enough because I write col1, col2, ..., colN twice. It will be painful if I want to update the selected columns.
Does anybody have any other suggestion?
select * from A
union all CORRESPONDING BY (col1, col2, ..., colN)
select * from B
ANSI/ISO SQL-2016, Feature F301, CORRESPONDING in query expressions.
I am using a platform which accepts minimal SQL functions to write a SQL code. The UNPIVOT function cannot be used on the platform so I have to do this manually. I am thinking along the line of UNION ALL and then CROSS JOINING (which I attempted but ended up with the wrong record counts. Please see image attached.
Any help / pointer will be highly appreciated!
I don't know how you used UNION ALL but it can be done like this:
select col1, col2, col3 as NewCol from Table1
union all
select col1, col2, col4 from Table1
You could also use an ORDER BY clause, so that rows with the same col1 and col2 appear in subsequent rows:
select col1, col2, NewCol
from (
select col1, col2, col3 as NewCol, 1 as ord from Table1
union all
select col1, col2, col4, 2 from Table1
) t
order by col1, col2, ord
A portable approach uses union all:
select col1, col2, col3 as newcol from mytable
union all
select col1, col2, col4 from mytable
If your database supports lateral joins (also called cross apply in some databases) and values(), this can be simplified:
select t.col1, t.col2, x.newcol
from mytable t
cross join lateral (values(col3), (col4)) x(newcol)
You can use a cross join, but it requires some case logic. The exact syntax depends on the database, but something like this:
select t.col1, t.col2,
(case when n.n = 1 then t.col3 else t.col4 end) as newcol
from t cross join
(select 1 as n union all select 2) n;
To load another table, you would do one of the following:
insert these results into a table that has already been created.
Use select into or create table as depending on the database.
If you care about the ordering, then you can add order by t.col1, t.col2, n.n.
In most cases, a simple union all approach is fine (such as GMB suggests). That approach requires scanning the table twice, which incurs some additional overhead. However, if the "table" is really a complex query or view, then only processing it once is a bigger advantage.
I'm trying to write a query that pulls data from a lot of tables, and has about 20 unions. It's pulling the same information repeatedly, but with more layers each time, to show a sort of tree.
I want to compare the final two columns. I'm using a case to do this, and if I add a case to this query then I get the error "query block has incorrect number of results columns". This seems to be because the final select in the union has an extra column (the compare case).
Is there any way to work around this? I don't want to add the case to each select, as this would add about 15 more columns that I don't want.
Use a sub-query:
SELECT col1,
col2,
CASE
WHEN col1 = 'somevalue'
THEN 'someresult'
ELSE 'otherresult'
END AS col3
FROM (
SELECT col1, col2 FROM table1 UNION ALL
SELECT col1, col2 FROM table2 UNION ALL
SELECT col1, col2 FROM table3
-- ...
);
Or use a sub-query factoring clause:
WITH data ( col1, col2 ) AS (
SELECT col1, col2 FROM table1 UNION ALL
SELECT col1, col2 FROM table2 UNION ALL
SELECT col1, col2 FROM table3
-- ...
)
SELECT col1,
col2,
CASE
WHEN col1 = 'somevalue'
THEN 'someresult'
ELSE 'otherresult'
END AS col3
FROM data;
Experts, I have a single table with multiple columns. col1, col2, col3, col4, col5, col6
I need to select distinct (col4), but I need all other columns also on my output.
If I run, this ( select distinct(col4 ) from table1 ), then I get only col4 on my output.
May I know, how to do it on db2?.
Thank you
You simply do this...
Select * From Table1 Where col4 In (Select Distinct(col4) From Table1)
I'm not sure if you will be able to do this.
You might try to run group by on this column. You will be able to run some aggregate functions on other columns.
select count(col1), col4 from table1 group by (col4);
none of the answers worked for me so here is one that i got working. use group by on col4 while taking max values of other columns
select max(col1) as col1,max(col2) as col2,max(col3) as col3
, col4
from
table1
group by col4
At least in DB2, you can execute
SELECT
DISTINCT *
FROM
<YOUR TABLE>
Which will give you every distinct combination of your (in this case) 6 columns.
Otherwise, you'll have to specify what columns you want to include. If you do that, you can either use select distinct or group by.
I am trying to create a new table from the result of a select. This works fine with SQL Server:
SELECT * INTO newTable FROM (SELECT col1, col2, col3 FROM oldTable) x;
Now, I want to achieve the exact same thing with HSQLDB (Version 2.2). I have tried several forms like
SELECT * INTO newTable FROM (SELECT col1, col2, col3 FROM oldTable);
SELECT INTO newTable FROM SELECT col1, col2, col3 FROM oldTable;
CREATE TABLE newTable AS SELECT col1, col2, col3 FROM oldTable;
All these variants result in some form of syntax error. How can I create a table from a select with HSQLDB?
The manual has an example for this:
CREATE TABLE t (a, b, c) AS (SELECT * FROM atable) WITH DATA
HSQLDB requires parentheses around the select (unlike all other DBMS) and it also requires the WITH DATA clause
Ok I found very easier way to do this.
select * into t_bckp FROM t;
Its interesting.